Thermo-catalytic methane decomposition over unsupported Fe–Al and Co–Al catalysts for hydrogen and carbon nanostructures production

The effect of aluminum as a promoter for cobalt and iron catalysts was investigated. The unsupported Fe–Al and Co–Al catalysts were synthesized by co-precipitation, and their catalytic activity in methane decomposition was evaluated. The physiochemical properties of Fe–Al and Co–Al were thoroughly investigated by various techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET and Raman spectroscopy. Fe–Al has a larger pore surface area than Co–Al and the morphology of the two is different. The Co–Al sample contains cobalt aluminate and cobalt oxide. The Fe–Al sample on the other hand, consists of Fe₂O₃ (65.4%) and hercynite FeAl₂O₄ (34.5%). The XPS analysis confirms the presence of the different oxides Fe³⁺, Fe²⁺, Co²⁺, Co³⁺ and Al₂O₃ on the surface of the samples. Raman results indicate the cation distribution between tetrahedral and octahedral sites. The Fe–Al catalyst shows a higher catalytic activity for methane conversion (95%), a hydrogen formation rate of 176.10 × 10⁻⁵ mol H₂ g⁻¹min⁻¹, and no sign of deactivation during 300 min. In contrast, Co–Al deactivated rapidly after ∼30 min. TEM images of spent catalysts show the presence of various forms of deposited carbon: carbon nano-onions (CNOs), multi-walled carbon nanotubes (MWCNTs) and bamboo-like carbon nanotubes (BCNTs).